Morphometric analysis of the conodont <i>Chiosella timorensis</i> (Nogami) from the early Anisian of Romania and China, and its significance for the definition of the Olenekian–Anisian Boundary

Abstract The conodont Chiosella timorensis (Nogami) has long been proposed as a biotic proxy for identifying the Olenekian–Anisian Boundary (OAB). However, uncertainty regarding the relationship between this species and the morphologically related species Ch . gondolelloides (Bender) and its stratigraphic range have cast doubt on its suitability as a proxy. Morphometric analysis of large populations of Chiosella from the global stratotype section and point (GSSP) candidate at Deşli Caira, southeastern Romania, as well as other OAB sections in south China, demonstrate that most morphological variation in this genus is caused by variability in the position of the cusp, not the length of the mid‐lateral rib (traditionally used to separate the species Ch . gondoelloides and Ch . timorensis ). Furthermore, there is little to no stratigraphic separation between the two species, and no obvious trend in any of the investigated sections of a morphological shift from one species to the other with increasing stratigraphic height. Finally, when investigating the size variation of the population, it becomes apparent that in general, specimens assigned to Ch . gondolelloides are much smaller than those Ch . timorensis . Taken together, these results imply that most specimens of Ch . gondolelloides could be juvenile forms of Ch . timorensis . If Ch . timorensis is used as the biotic proxy for the OAB, then the results of this morphometric analysis necessitate the base of the Anisian being moved down in several OAB sections, including Deşli Caira. Despite this, the study provides greater taxonomic stability, enhancing the utility of Ch . timorensis for future stratigraphic correlation.